A ketogenic diet -- which provides 99% of calories from fat and only 1% from carbohydrates -- produces health benefits in the short term, but negative effects after about a week, researchers found in a study of mice.

The results offer early indications that the keto diet could, over limited time periods, improve human health by lowering diabetes risk and inflammation. They also represent an important first step toward possible clinical trials in humans.

The keto diet has become increasingly popular as celebrities, including Gwyneth Paltrow, Lebron James, and Kim Kardashian, have touted it as a weight-loss regimen.
In the study, published in Nature Metabolism, researchers found that the positive and negative effects of the diet both relate to immune cells called gamma delta T-cells, tissue-protective cells that lower diabetes risk and inflammation.

A keto diet tricks the body into burning fat, said the lead author. When the body's glucose level is reduced due to the diet's low carbohydrate content, the body acts as if it is in a starvation state -- although it is not -- and begins burning fats instead of carbohydrates. This process in turn yields chemicals called ketone bodies as an alternative source of fuel. When the body burns ketone bodies, tissue-protective gamma delta T-cells expand throughout the body.

This reduces diabetes risk and inflammation, and improves the body's metabolism, said the author. After a week on the keto diet, mice show a reduction in blood sugar levels and inflammation.

But when the body is in this "starving-not-starving" mode, fat storage is also happening simultaneously with fat breakdown, the researchers found. When mice continue to eat the high-fat, low-carb diet beyond one week, they consume more fat than they can burn, and develop diabetes and obesity.

"They lose the protective gamma delta T-cells in the fat," the author said.

Long-term clinical studies in humans are still necessary to validate the anecdotal claims of keto's health benefits.

"Before such a diet can be prescribed, a large clinical trial in controlled conditions is necessary to understand the mechanism behind metabolic and immunological benefits or any potential harm to individuals who are overweight and pre-diabetic," the author said.

There are good reasons to pursue further study: According to the Centers for Disease Control, approximately 84 million American adults -- or more than one out of three -- have prediabetes (increased blood sugar levels), putting them at higher risk of developing type 2 diabetes, heart disease, and stroke. More than 90% of people with this condition don't know they have it.

"Obesity and type 2 diabetes are lifestyle diseases," the author said. "Diet allows people a way to be in control."

With the latest findings, researchers now better understand the mechanisms at work in bodies sustained on the keto diet, and why the diet may bring health benefits over limited time periods.

"Our findings highlight the interplay between metabolism and the immune system, and how it coordinates maintenance of healthy tissue function," said the lead author.

https://www.nature.com/articles/s42255-019-0160-6